1. Field of the Invention
The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for precompensation value determination in a PRML data channel.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Partial-response signaling with maximum-likelihood sequence detection techniques are known for digital data communication and recording applications. Achievement of high data density and high data rates has resulted in the use of a PRML channel for writing and reading digital data on the disks.
Nonlinear bit shift (NLBS) in magnetic recording is the shift in position of a written transition due to the proximity effect of a preceding transition. In PRML, the readback waveform is synchronously sampled at regular intervals. Sample values depend on the position of written transitions. Therefore an unwanted shift, such as a nonlinear bit shift, leads to error in sample values which, in turn, degrades the performance of the PRML channel. Write precompensation or precomp is employed to counteract NLBS in the magnetics by shifting the transition during writing. However, known methods for measuring the NLBS and adjusting the write precomp add complexity to the PRML channel.
For example, one proposed method for measuring NLBS is to define a dibit response with two isolated transitions and identify their positions. Another proposed method uses a specially designed psuedo-random sequence to determine an impulse response. The bit shift is then measured by an echo at a known position. Both methods require complicated data manipulation and high precision waveform recording. A third method identifies the Volterra kernels of a nonlinear system. These methods are relatively complex and slow. A fourth method uses a special bit pattern and a frequency selective receiver to measure the NLBS. The bit pattern is designed to eliminate predefined frequency components when all bits are written without any NLBS and the magnitude of the frequency when detected can be used to adjust the write precomp to compensate for NLBS. Hardware not normally present in a PRML channel is required for the fourth method.
Disadvantages of the above-described and other known arrangements include both the additional hardware and time required for performing the write precompensation adjustment methods.